Orthogonal Frequency-Division Multiple Access (OFDMA) is a multiple access method for sharing a radio frequency (RF) channel among multiple users. OFDMA uses an orthogonal frequency-division multiplexing (OFDM) digital modulation scheme to modulate information signals. OFDMA can be described as a combination of frequency domain and time domain multiple access. In OFDMA, a communication space is divided into a plurality of time slots, and each time slot is further divided into a number of frequency sub-channels each having at least one of its own sub-carriers. In OFDMA systems, both time and/or frequency resources are used to separate multiple user signals. Transmissions to/from multiple users are separated using time slots and sub-channels within each time slot such that users' signals can be separated in the time domain and/or in the frequency domain. Thus, in OFDMA, resources can be partitioned in the time-frequency space.
Recently, broadband wireless networks have been developed that implement OFDMA. For instance, IEEE 802.16 networks are one example. As used herein, “IEEE 802.16” refers to a set of IEEE Wireless LAN (WLAN) standards that govern broadband wireless access methods. IEEE 802.16 standards have been and are currently being developed by working group 16 of the IEEE local area network/metropolitan area network (LAN/MAN) Standards Committee (IEEE 802). Any of the IEEE standards or specifications referred to herein may be obtained at http://standards.ieee.org/getieee802/index.html or by contacting the IEEE at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA. The Institute of Electrical and Electronics Engineers (IEEE) 802.16 Working Group on Broadband Wireless Access Standards is a unit of the IEEE 802 LAN/MAN Standards Committee that aims to prepare formal specifications to support the development and deployment of broadband Wireless Metropolitan Area Networks. In such 802.16 communication networks, communications signals between a base station and a station are modulated using OFDM. In one configuration, the OFDMA channel is split into a number of time slots. Each time slot is further divided into a number of frequency sub-channels (e.g., one 70 MHz wide time slot can be divided into fourteen sub-channels each being five MHz wide).
Near-Far Problem
In a wireless communication system, the near-far problem refers to the situation where a receiver receives a low-power signal from a transmitter and a high-power signal from a different transmitter at the same time, resulting in desensitization or “desense” of the receiver to the low-power signal. In other words, the high-power signal may cause the low-power signal to fall below the receiver's detectability threshold. For instance, when a high power transmitter is located near a receiver operating in the same time slot but on a different frequency sub-channel, the high transmit energy can desensitize the receiver.
Scheduling
Scheduling algorithms are widely used in wireless networks for allocating or distributing communication resources (e.g., time slots and/or sub-channels) among stations to take advantage of instantaneous channel variations by giving priority to the stations with favorable channel conditions. For instance, in an OFDMA communication system, the base station can include a time-division multiple access (TDMA) scheduler that schedules time/frequency resources used by each normal uplink communication and each downlink communication. A normal uplink communication is when a station transmits to a base station and downlink communication when the base station transmits to a station. A scheduler may assign an uplink communication on different sub-channels within the same time slot to different stations. In particular, the base station scheduler may schedule these uplink communications either in different time slots or in the same time slot and uses power control to prevent/reduce near-far interference among various stations communicating to the base station in the system. The TDMA scheduler avoids near-far problems by creating time-orthogonal uplink and downlink transmissions, and through uplink power control. For example, in OFDMA solutions which have a base station centric deployment, such as IEEE 802.16, the near-far problem is reduced by forcing each base station to create either time (TDD) or frequency (FDD) orthogonal uplinks and downlinks to prevent desensitization of the mobile receiver. Power control of uplink transmissions from a mobile station can be used to assure that signals arrive at the base station receiver at similar power levels thereby preventing desensitization of the base station's receiver.
Thus, in networks such as these, in which stations communicate directly with a base station using orthogonal frequency-division multiple access (OFDMA) for the uplink, such TDMA scheduling techniques can be used to separate low-power and high-power users in time to avoid near-far problems. However, as will be described below, such TDMA scheduling techniques will not work in all types of OFDMA networks.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.